IELTS (International English Language Testing System) is a complex English language skills examination which tests the applicants' academic or general knowledge of English.
The exam focuses on testing the applicants' receptive and productive skills both (ie reading and listening, speaking and writing respectively).
There are two distinctly different IELTS exam versions:

Academic IELTS is aimed at secondary school leavers who would like to continue their studies at an English speaking university or college, and thus need to prove they have gained
a satisfactorily good command of English.
General IELTS is aimed at professionals who need a formal qualification to prove their English skills, for instance, to their employers. Moreover, the general version focuses on people
migrating into English speaking countries and foreign students who are interested in taking up studies at an English speaking secondary school.
If you are interested in taking the exam, you might want to visit the official IELTS website.
For more information on IELTS (including preparation for Writing -- Graphs and Charts Descriptions), you might want to visit the following links:
1 Tips for Taking IELTS                                          6 British Council Resources
2 General Information                                              7 British Council Resources Cont.
3 IELTS Exam Preparation                                      8 British Council Resources Cont.
4 Descriptions Intro                                                 9 British Council Resources Cont.
5 Charts, Graphs and Diagrams                               10 British Council Resources Cont.